We report the first observation of laser cooling in 1% doped Tm:YLF by 0.5 K and in 0.8% doped Ho:YLF crystals by
0.1 K starting from room temperature in air. To achieve this, we designed and constructed a high power, broadly tunable
(1735 nm-2086 nm) continuous wave singly-resonant optical parametric oscillator. (OPO). The cooling experiments were
performed at ambient pressure, and temperature changes were measured using a thermal camera.

Optical cooling of solids is a promising and innovative method to provide cryogenic cooling to infrared sensors. Currently insulator crystals, specifically ytterbium-doped yttrium- lithium-fluoride (Yb:YLF), have shown the most promise for cooling to low temperatures. This method has demonstrated cooling below the National Institute of Standards and Technology (NIST) cryogenic temperature definition of less than 123 K. Optical refrigeration utilizes a phenomenon called anti-Stokes fluorescence to generate cooling power. Incident laser light is absorbed by the cooling crystal and photons are spontaneously emitted at a higher, and thus more energetic, frequency. The difference in frequency is proportional to the cooling power of the crystal. Anti-Stokes cooling is highly dependent on doping percentages and YLF crystal purity and structure. Space based infrared sensors and their coolers are operated in a radiation environment where protons, gamma, rays, heavy ions, and other radiation species are common and of varying severities depending on operational orbit. To ensure that radiative effects on cooling crystal performance are minimal, we irradiated two samples with 63 MeV protons to a total of ionized dose of 100 Krad (Si) and 1 Mrad (Si), and compared cooling crystal efficiency parameters before and after dosing.

Laser cooling in InGaP|GaAs double heterostructures (DHS) has been a sought after goal. Even though very high external quantum efficiency (EQE) has been achieved, background absorption has remained a bottleneck in achieving net cooling. The purpose of this study is to gain more insight into the source of the background absorption for InGaP|GaAs DHS as well as GaAs|AlGaAs DBRs by employing an excite-probe thermal Z-scan measurement.

Laser cooling of solids has great potential to achieve an all-solid-state optical cryo-cooler. The advantages of compactness, no vibrations, no moving parts or fluids, and high reliability have motivated intensive research. Increasing the pump power absorption is essential to reach lower temperatures. Here, using a high power broadly tunable InGaAs/GaAs vertical external-cavity surface-emitting laser (VECSEL) we demonstrate how we have increased the pump power absorption in an intra-cavity geometry cooling a 10% Yb:YLF crystal. We also discuss the progress, advantages, and challenges of laser cooling inside a VECSEL cavity, including the VECSEL active region design, cavity design, and cooling sample choice for optimal cooling. A novel method to increase the absorption of the pump power in the crystal has also been proposed.

Solid-state optical refrigeration is an emerging cooling technology that can provide vibration free and reliable refrigeration to cryogenic temperatures in a lightweight and compact device. The technology has matured over the past two decades and is currently being considered for applications where the mechanical vibrations, limited reliability, or insufficient portability of existing cooling technologies pose challenges. Possible applications include satellite-borne infrared imaging, laser metrology, and gamma-ray spectroscopy as well as high-reliability cooling of semiconductors and high-temperature superconductors. The best results achieved so far have been in cooling rare-earth-doped solids, especially materials doped with ytterbium. We discuss the fundamental physical principles of solid-state laser cooling, the resulting material and device design requirements, and the estimated payload heat lift of an optical cryocooler.

Laser cooling of Yb:YLF crystal to 131 K from room temperature has been demonstrated in an active intracavity arrangement for enhanced pump absorption. The laser is a high-power, broadly-tunable InGaAs/GaAs MQW VECSEL capable of producing 20 Watts at 1020 nm, directly at the E4-E5 transition of the Yb-ion. This is the coldest temperature achieved to date in an intracavity geometry and without sophisticated heat load management of the crystal. This progress presents a significant advancement towards an all-solid-state compact cryocooler.

With the coldest solid-state temperatures (ΔT <185K from 300K) achievable by optical refrigeration, it is now timely to apply this technology to cryogenic devices. Along with thermal management and pump absorption, this work addresses the most key engineering challenge of transferring cooling power to the payload while efficiently rejecting optical waste-heat fluorescence. We discuss our optimized design of such a thermal link, which shows excellent performance in optical rejection and thermal properties.

The role of transition-metal impurities in Yb3+-doped YLiF4 (YLF) laser-cooling crystals is studied. Divalent 3d transition-metal ions, in particular Fe2+, are found to have strong absorptions at the laser cooling pump wavelength and degrade the cooling efficiency by introducing background absorption. A set of eight substitutional and chargecompensated defects that form upon introduction of 1+, 2+, and 3+ transition-metal ions into the YLF crystal lattice is proposed. A calculation of solution energies for each defect type and for a range of 3d ions is carried out. It indicates that divalent 3d ions preferentially substitute for Y3+ accompanied by a fluoride vacancy for charge compensation. An electron paramagnetic resonance (EPR) study of a YLF crystal identifies Fe2+ in the crystal lattice, in agreement with the elemental analysis and the computational results. A strategy for purifying the YF3, LiF, and YbF3 starting materials for the YLF:Yb crystal growth is discussed. Chelate-assisted solvent extraction purification with pyrrolidine dithiocarbamate (APDC) for Y, Li, and Yb as well as ethylenediaminetetraacetic acid (EDTA) for Li was carried out.

We have achieved cryogenic optical refrigeration with a record low temperature in optical refrigeration by cooling 5% wt.Yb:YLF crystal to 119K ±1K (~-154 C) at 1=1020 nm corresponding to its E4-E5 Stark manifold resonance with an estimated cooling power of 18 mW. This demonstration confirms the predicted minimum achievable temperature (MAT). Further cooling is achievable as shown by measurements of a doping study where a 10% wt. Yb:YLF crystal with reduced parasitic heating has predicted cooling below 100K (~-173K).

We report on the use of a high power InGaAs quantum well vertical external-cavity surface-emitting laser (VECSEL) emitting at a wavelength of 1020 nm for intra-cavity cooling of a 5% Yb-doped YLF crystal to 148 K from room temperature. Similar crystals have now reached temperatures below the NIST-defined cryogenic temperature of 123 K when pumped outside a laser cavity. We discuss the progress, advantages, and challenges of laser cooling inside a VECSEL cavity, including the VECSEL active region design, cavity design, and cooling sample choice for optimal cooling.

Laser cooling of solids to 148 K has been demonstrated in a Yb:YLF crystal using intracavity absorption enhancement in
an InGaAs MQW VECSEL at 1020 nm. This is the lowest temperature achieved in the intracavity geometry to date and
presents a significant advancement towards an all-solid-state compact cryocooler.

Optical refrigerator is based on an optical heat removal from a cooling element by
satisfying conditions of anti-Stokes fluorescence. This imposes a challenge of optically shielding a
payload from the anti-Stokes radiation. Therefore, a link between the cooling element and a
payload (the thermal link) has to exhibit high thermal conductivity while blocking the
fluorescence and scattered laser light in a non-absorbing manner. In this paper we examine photon
blockade structures ranging from textured kinks to hybrid systems involving light-shedding
geometries and semiconductor-based epitaxially-grown distributed Bragg mirrors.

Since recent demonstration of cryogenic optical refrigeration, a need for reliable
characterization tools of cooling performance of different materials is in high demand. We present
our experimental apparatus that allows for temperature and wavelength dependent characterization
of the materials' cooling efficiency and is based on highly sensitive spectral differencing technique
or two-band differential spectral metrology (2B-DSM). First characterization of a 5% w.t.
ytterbium-doped YLF crystal showed quantitative agreement with the current laser cooling model,
as well as measured a minimum achievable temperature (MAT) at 110 K. Other materials and ion
concentrations are also investigated and reported here.

Based on a highly sensitive dierential spectroscopy technique, we present a non-contact method of optical-
scanning thermal imaging with a possibility of sub-thermal-wavelength spatial resolution. This technique is
general and can also be applied to imaging of strain or impurity distributions at the surfaces of semiconductors.
This procedure is particularly well suited for near-eld imaging and investigation of thermal transport on the
nanoscale. Applications to optical refrigeration in semiconductors are discussed.

We utilize highly sensitive spectroscopic local temperature probe to ascertain cooling performance of Yb:YLF
crystal as a function of wavelength and temperature. A minimum achievable temperature of 120 K is measured
at pump wavelength corresponding to E4-E5 Stark manifold transition. Results verify current model for laser
cooling cycle as well as demonstrate the lowest temperature achievable by means of optical refrigeration to date.

We demonstrate first cryogenic operation in a Ytterbium doped YLF crystal by means of an optical refrigeration.
We have achieved cooling to 155 Kelvin absolute temperature with heat lift of 90 mW, exceeding performance of
multi-stack thermo-electric coolers. This progress was possible by pumping the system near the Stark-manifold
resonance of highly pure Yb:YLF crystal and careful thermal management in the cooling experiment. Detailed
spectroscopic analysis demonstrated that cooling to 110 Kelvin is currently possible if pumped exactly on that
resonance.

Spectroscopic characterization of YLF crystal doped with Yb reveals the performance potential of this material in laser
cooling applications. Temperature-dependent spectra allow us to estimate the minimum achievable temperature and the
parasitic background absorption.

We discuss recent progress in the laser cooling experiments via resonant cavity. Following analysis of the cooling
efficiency, we highlight importance of wavelength dependence of the minimum achievable temperature for a given
cryocooler. Following the analysis, we utilize pump detuning along with reduction of thermal load on the sample
to achieve absolute temperature of nearly 200K, a 98.5 degree drop, starting from room temperature. Wavelength
dependent analysis suggests that further improvement is possible.

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